Traffic on California highways over the Labor Day holiday is expected to be 21% higher than a normal weekend, according to the state Department of Transportation.
The forecast compares to a 13% jump last year during the four-day holiday. Travel forecasters believe this year's heavy increase partly reflects the state's economic recovery.
Caltrans traffic engineers estimated that motorists will travel 344 million additional miles between Friday and Monday.
"Labor Day is traditionally one of the heaviest-traveled weekends of the year," Caltrans spokesman Jim Drago said. "This year is no exception. We are urging all travelers to allow plenty of time for their trip, especially to and from the popular tourist destinations."
Caltrans maintains a system of electronic detectors throughout the state to count cars on the freeways. Traffic engineers based their forecast on a sampling of bellwether count stations representing every region of the state.